What is a shading unit and a texture mapping unit in a GPU ? and how does it affect blender?
These statistics do not impact blender directly. You can have thousands of nodes and shaders and terrabytes of textures if you want and if your PC can handle it. 

This is where these components come back into play. The better a GPU is the better it can handle those things. Your GPU can't hold terrabytes of textures, and you're probably going to kill your rendertimes with a nodes setup thousands of nodes deep. 

The units mentioned here are directly related to the architecture of the card. A texture mapping unit for example is what's actually doing the caculations for UV unwrapping. Having less of them doesn't change the size of your possible UV map, it just means it takes longer to calculate the unwrapping.